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The first six weeks of pregnancy, explained

If a woman is considered pregnant at six weeks, she would have a maximum of two weeks to realize this.

That's because a fetus's gestational age is counted not from the time sperm fertilizes an egg or from the time you have a positive pregnancy test, but weeks before, on the first day of the previous menstrual cycle. That means a woman is six weeks pregnant just two weeks after missing her period, Dr. Dawnette Lewis, director of the Center for Maternal Health at Northwell Health in New York and a specialist in maternal and fetal medicine.

This applies if someone has a typical menstrual cycle that lasts about four weeks. But several factors — including stress, perimenopause and certain health conditions — can make the menstrual cycle so unpredictable that it can take longer than six weeks for someone to realize they might be pregnant.

“People come in and say, 'I've always had irregular periods and just thought I'd gain weight,' and lo and behold, they're pregnant,” said Dr. Shruthi Mahalingaiah, fertility doctor at Massachusetts General Hospital and professor of environmental and reproductive health at Harvard TH Chan School of Public Health.

The question of how quickly it is possible to detect and confirm a pregnancy has come into focus as new abortion restrictions have been enacted in 21 states, including Florida, which passed a ban on most abortions after six weeks on May 1 came into force. Florida is the third state to ban abortions at this point in pregnancy.

We asked experts to explain what the first six weeks of pregnancy are like and what factors could make it difficult to detect pregnancy.

If an egg is not fertilized, the uterus sheds its lining. The first day of bleeding is considered day 1 of a menstrual cycle. Typically, this bleeding lasts between three and seven days, Dr. said. Lewis.

When bleeding subsides, estrogen and another hormone called follicle-stimulating hormone, or FSH, begin to increase. Together, they stimulate the ovaries to produce a mature egg and create a fresh uterine lining for a potentially fertilized egg.

On or around day 14 of a cycle, the ovary would normally release an egg. If an egg is released and sperm are present, fertilization in the fallopian tube can occur one to two days after ovulation.

If an egg has been fertilized, what Normally it travels into the uterus and implants itself in the uterine lining, where it can become an embryo. This can take about a week or longer, Dr. Michael Belmonte, obstetrician and gynecologist and fellow at the American College of Obstetricians and Gynecologists.

Once this happens, the body begins to produce a hormone called human chorionic gonadotropin, also known as the pregnancy hormone. This tells the ovaries to stop producing another egg.

At implantation, a woman is technically considered to be around four weeks pregnant based on the gestational age calculation. But she probably wouldn't have any physical pregnancy symptoms like nausea or fatigue. HCG levels may also be too low to be detected with a home pregnancy test. it wasn't detectable until about a week after implantation, said Dr. Belmonte.

It is impossible to know whether the embryo is developing normally or whether it has implanted in the uterus at all, said Dr. Mahalingaiah. About half of fertilized embryos do not implant at all. In an estimated 2 percent of pregnancies, the embryo implants somewhere else, such as the fallopian tube. This is called an ectopic pregnancy and can be life-threatening and require medical or surgical intervention.

Roughly speaking, if you are pregnant, this is the earliest time you can detect pregnancy at home. If you have a regular four-week cycle, have missed your period, and think you may be pregnant, you could do a home urine test at the end of that week, which would detect elevated hCG levels.

However, there are several reasons to expect irregular periods and not to think about taking a pregnancy test at this time. Polycystic ovary syndrome, which affects up to five million women in the United States, can disrupt ovulation and cause irregular menstrual cycles, as can other endocrine disorders such as thyroid disease. Significant stress in the first two weeks of your cycle can delay ovulation or menstruation, Dr. Mahalingaiah, as well as a bad case of the flu. Those in perimenopause, the transition to menopause, experience increasingly irregular ovulation as they approach the end of their reproductive years. This means that the length of your menstrual cycle can change every month, often by several weeks.

Even if you suspected you were pregnant at about 5 weeks, a health care provider would not be able to confirm that pregnancy is possible using an ultrasound because there is no fetal cardiac activity at that time. It could also turn out that the pregnancy was a so-called biochemical pregnancy, said Dr. Mahalingaiah. In this case, someone may test positive on an early pregnancy test at home, but the fetus may not continue to develop. This results in pregnancy loss that simply looks like a late period. Some research suggests that an estimated 22 percent of pregnancies end this way.

This is about the time an ultrasound could detect the fetus's cardiac activity, which may sound like a heartbeat. Some states require an ultrasound scan before a woman can have an abortion, including a medication abortion. Florida requires two in-person visits 24 hours apart, including an ultrasound. The new law also prohibits telemedicine providers from prescribing abortion medications.

This point in time, said Dr. Belmonte, leaves women “a very small window of opportunity” in which to confirm a pregnancy and have an abortion.

Anna Harden

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